Various products or methods exist and are already utilized for these various uses. Treatments using polymers are known in particular in the field of water inflow prevention, but they remain limited to relatively low water-producing zone permeabilities (on average below 300 mD) because of the size limitation of these polymers.
The water inflow prevention methods based on polymer gels provided in the petroleum industry are currently not very reliable and often use polluting products based on chromium salts or resins. The methods currently on the market most often use plugging gels or diluted gels (low polymer and crosslinking agent concentration, colloidal dispersion gels), the results remain uncertain mainly because of problems such as:                absence of control of the gelation kinetics linked with the variability of the physico-chemical and hydrodynamic parameters between the surface and the formation, involving a risk of treatment inefficiency (absence of gelation) or, conversely, a risk of irreparable well damage (gel setting),        retention and adsorption of the crosslinking agent on the reservoir rock,        non-control of the characteristics of the gel formed, of its positioning, of its propagation and of its water/oil selectivity properties.        
On the other hand, the increasing number of mature fields, the development of complex wells (horizontal, subsea, multibranch wells) and the reliability problems linked with bottomhole separation techniques have led oilmen to take an active interest in self-selective water inflow prevention treatments that can be injected directly (bullhead type injection) into the reservoir formation, i.e. without zone isolation by means of specific equipments.
When a producing well is treated by polymer injection, the primary mechanism is based on an adsorption of the polymer molecules on the mineral making up the layer swept by the polymer. After treating, when the well produces again, the molecules nearly irreversibly adsorbed on the wall afford a maximum amount of resistance to the flow of the water circulating mainly in the vicinity of this wall and thus reduce its mobility. In the presence of a hydrocarbon phase (oil or gas), the capillary pressure exerted by the fluid making up this phase is sufficient to crush the molecules adsorbed on the walls of the pore restrictions and thus not to hinder its mobility. The molecules, deformable in their “compressed” state, have the capacity to swell in the presence of a water phase and thus to reduce its mobility. This type of product providing a selective permeability reduction between water, oil and gas is referred to as “Relative Permeability Modifier”, it thus involves a reduction of the water permeability in the drain holes of higher permeability, therefore favouring:                1) production improvement (Water-Oil Ratio or Gas-Oil Ratio decrease),        2) more efficient sweep of the layers of lower permeability, in general those containing still mobilizable oil,        3) diversion of the water to the less permeable zones which are therefore less swept in the drainage methods using water injection (profile correction in injection wells).        
A microgel preparation method is described in document EP-1,086,976. It is based on gelation performed under shearing of a mixture of polymer and of crosslinking agent. According to the method, a gelling composition comprising a polymer and a polymer crosslinking additive is injected into a porous and permeable medium, and a mono-disperse solution of microgels of substantially constant size is recovered at the outlet.
The applicant has thus taken an interest in chemical species easy to pass into solution, stable, i.e. having a repellent character (absence of interactions), and of controlled size.
The present application relates to a method using microgels of chemical type, of controlled and stable size, optimized according to the reservoir characteristics for which the application is desired.
The advantages of this invention mainly lie in the fact that:                a) the size of the microgels is notably larger (1 to 10 times) than the size of a commercial polymer of high molecular weight,        b) the size of the microgels can be dimensioned according to the mean permeability of the layer or of the reservoir zone by which the water is produced in larger amounts, generally zones with a permeability ranging between 0.001 and some Darcy (1 Darcy=0.98693 μm2).        
In the present invention, the applicant has sought microgels having the following properties:                deformable,        stable in solution,        stable towards electrolytes,        temperature stable,        mechanically stable, i.e. when subjected to strong shearing,        capable of irreversible adsorption.        
The capacity of the microgels according to the invention to meet in the widest range all of these conditions allows to provide a new method, much more reliable than those currently available on the market.
According to the invention, the microgels are non-toxic, without harmful residues, thus allowing to satisfy the evolution of the European regulations on dangerous substances and standards on emissions.